STRATIGRAPHIC CORRELATION, FOSSILS, FACIES, AND SEA LEVEL CHANGE

Correlation of Strata

The need to classify and organize rock layers according to relative age led to the geologic discipline of stratigraphy.

Rocks at different locations on Earth give different "snapshots" of the geologic time column.  At a particular location, the rocks never fully represent the entire geologic rock column due to extensive erosion or periods of non-deposition or erosion.

The thickness of a particular rock layer (representing a particular time period) will vary from one location to another or even disappear altogether.

The process that stratigraphers use to understand these relationships between strata at different localities is known as "correlation".

For example, rocks named Juras (for the Juras Mountains) in France and Switzerland were traced northward and found to overlie a group of rocks in Germany namedTrias.  The Trias rocks in turn, were found to underlie rocks named Cretaceous in England (the chalky “White Cliffs of Dover”).

Based on these relationships, is the Juras older or younger than the Cretaceous?  What are the two possible scenarios?

The location where a particular rock layer was discovered is called a "type locality".  Most of the “type localities” of the geologic time column are located in Europe because this is where the science of stratigraphic correlation started.

The Sedgwick/Murchison Debate

In 1835, Adam Sedgwick (Britain) and Roderick Murchison (Scotland) decided to name the entire succession of sedimentary rocks exposed throughout Europe.  They were geology colleagues and friends, but they had a famous argument over the division between the Cambrian and Silurian in Wales. 

Sedgwick’s topmost Cambrian overlapped with Murchison’s lowermost Silurian.  Eventually the disputed rock layers were assigned the age “Ordovician”.

Rocks Divisions versus Time Divisions

It is important to remember that the rock record is an incomplete representation of real geologic time due to the presence of unconformities.

Therefore, geologists are careful to distinguish geologic time from the rocks that represent snapshots of geologic time:

TIME DIVISIONS	CORRESPONDING ROCK DIVISIONS (AND ROCK UNITS)
Eon Examples: Precambrian/Phanerozoic	Eonothem
Era           Examples: Paleozoic/Cenozoic/Mesozoic	Erathem
Period                Examples: Cambrian/Ordovician/Silurian	System               Groups                     Formations (The main stratigraphic unit)                          Members

Rock divisions, such as the Cambrian System, can be correlated worldwide based on fossils.  In contrast, rock units such as groups, formations, and members are localized subsets of systems.  Rock units depend on the environment of deposition, which varies from one location to another.

Stratigraphic Rock Units

The rock divisions (Eonothem, Erathem, and System) simply divide rocks into the appropriate time eon, era, or period.  Obviously, all Cambrian System rocks are from the Cambrian regardless of their location on Earth's surface.

In contrast, the rock units (Groups, Formations, Members) are localized features (of limited regional extent) that depend on the local environment of deposition. 

The main rock unit of stratigraphy is the formation, a localized and distinctive (easily recognizable) geologic feature (i.e., the Chinle Formation of Late Triassic lake and river deposits in Arizona, Nevada, Utah, and New Mexico).

Different formations are distinguished and correlated based upon lithology (overall rock characteristics), which includes:

1) Composition of mineral grains

2) Color

3) Texture (grain size, sedimentary structures)

4) Fossils

Formations are “clumped” into groups and divided into members.

Datum- In correlation, a datum is a line of equivalent age.

The ideal datum is a stratigraphic marker that is both geographically extensive and represents an instantaneous moment in geologic time.  A good example is a volcanic ash layer that formed by a specific volcanic eruption followed by worldwide dispersal by atmospheric currrents.

Using Fossils for Strata Correlation

Sedimentary rocks that date from the same age can be correlated over long distances with the help of fossils.

Principle of Fossil Correlation- Strata containing similar collections of fossils (called fossil assemblages) are of similar age.  Also, fossils at the bottom of the strata are older than fossils closer to the top of the strata.

Index Fossils- Index fossils are the main type of fossil used in correlation.  To be an index fossil, a fossil species must be:

1) Easily recognized (unique).

2) Widespread in occurrence from one location to another.

3) Restricted to a limited thickness of strata (limited in age range).

The limited life-spans of these organisms allows us to easily constrain the age of rocks in which they occur.

The best index fossils are those that are free floating and independent of a particular sedimentary environment.  For example, organisms that are attached to one particular type of sediment are going to have limited geographic extent and will not be found in many rock types.   By contrast, organisms that are “free floaters” or “swimmers” will have a wider geographic extent and be found in many different rock types (i.e., trilobites).

A fossil zone is an interval of strata characterized by a distinctive index fossil.

Fossil zones typically represent packets of 500,000 to 2,000,000 years.  Fossil zones boundaries do not have to correlate with rock formation boundaries.  Fossil zones may be restricted to a small portion of a formation or they may span more than one formation.

A fundamental assumption in fossil correlation is that once a species goes extinct, it will never reappear in the rock record at a later time.

Fossil types that are generally restricted to just one type of sediment are called facies fossils.  They are not very useful in correlation, but are extremely useful for reconstructing paleoenvironments.
  What is a Fossil?

Some examples of fossils are:

1) The preservation of entire organisms or body parts.  This includes the preservation of actual body parts (mammoths in tundra), as well as morphological preservation via the replacement of biological matter by minerals (petrified wood).

A petrified log in Petrified Forest National Park, Arizona, U.S.A.-impressions

2) Casts or impressions of organisms.

Eocene fossil fish Priscacara liops from Green River Formation of Utah

3) Tracks.

Trackways from ''Climactichnites'' (probably a slug-like animal), in the Late Cambrian of central Wisconsin.

4) Burrows.

Thalassinoides, burrows produced by crustaceans, from the Middle Jurassic of southern Israel.

5) Fecal matter (called coprolites).

Carnivorous dinosaur dung found in southwestern Saskatchewan,  USGS Image.

Theories on The Origin of Fossils

At one time, fossils were considered to be younger than the rocks in which they occurred.  People speculated that fossils formed when animals crawled into preexisting rock, died, and became preserved in stone.

Some people interpreted the widespread occurrence of fossilized marine organisms on land as support for a world-wide flood as described in scripture.

Leonardo da Vinci’s (1452 - 1519) Interpretation of Fossils

Self-portrait of Leonardo da Vinci, circa 1512-1515.

Regarding fossils that occur in strata many miles from the sea, da Vinci argued that:

1) The fossils could not have been washed in during a "Great Deluge" because they could not have traveled hundreds of miles in just 40 days.

2) The unbroken nature of the fossils suggest that they were not transported by violent water; instead the fossils represent formerly living communities of organisms that were preserved in situ.

3) The presence of fossil-rich strata separated by fossil-poor strata suggests that the fossils were not the result of a single worldwide flood, but formed during many separate events.

Lateral Variations in Formations

Historically, geologists initially believed that the layer-cake sequence of sedimentary rocks existed worldwide (i.e., that the layers extended indefinitely without change).

By the late 1700’s people began to realize that formations had a limited extent both vertically (up and down) and laterally (horizontally across Earth's surface).

People also began to realize that lithologic variations (changes in texture, color, fossils, etc) can occur laterally within formations themselves.

Today we interpret such variations in the context of modern depositional environments.  For example:

ENVIRONMENT OF DEPOSITION	EXPECTED LITHOLOGY
Near shore marine- The energy is high due to rough waters at the water-land interface.	Coarse sediments, and fossils of robust organisms that can withstand high energy environments.
Deep ocean- The energy is low due to the general calmness of water away from land.	Fine sediments, and fossils of more fragile organisms.
Note that the two different lithologies can be deposited simultaneously (representing the same moment in geological time) so long as they are deposited at different locations.

Different lithologies grade laterally into one another in a manner called intertonging.  An example is the way that the Old Red Sandstone of Wales (a terrestrial deposit) grades laterally into marine sediments of Devonshire to the south (both are Devonian).

Intertonging reflects the changes in depositional environments that occur over space and time (lateral and temporal variations).  Often these changes in environment are linked to shoreline migrations resulting from sea-level changes over time.

 Depositional Environments and Sedimentary Facies

Depositonal environments are characterized initially by the sediments that accumulate within them, and ultimately by the sedimentary rock types that form.  For example, a reef environment is characterized by carbonate reef-building organisms.  Ultimately, the sediments become lithified to form fossiliferous limestone.

A sedimentary facies is a three-dimensional body of sediment (or rock) that contains lithologies representative of a particular depositional environment.  For example,

FACIES	LITHOLOGIES
Floodplain	Mudstone and shale with interbedded sandstone.
Ocean basin	Laminated pelagic clays, cherts, and possible limestone.
Delta	Well-sorted, well-rounded, and possibly cross-bedded sandstone.

Analysis of sedimentary facies helps geologists to reconstruct past geologic environments and paleogeography.

Transgressions vs. Regressions

The sea-level has fluctuated throughout geologic history, and these changes have a profound effect on the geologic rock record.

A transgression is an advance of the sea over land.

A regression is a retreat of the sea from land area.

A transgressive facies pattern is characterized by:

1. The movement of marine facies landward over terrestrial facies.

2. A fining-upward sequence (the new marine environment is lower energy than the prior terrestrial environment).

3. A basal, erosional unconformity (erosion was more profound before the seas advanced).

A regressive facies pattern is characterized by:

1. The movement of terrestrial facies seaward and over marine facies.

2. A coarsening-upward sequence.

3. An erosional unconformity at the top.

Walther’s Law- Over time, the lateral changes in sedimentary facies due to transgressions and regressions will also produce vertical changes in sedimentary facies:

1. A transgressive facies sequence fines in the direction of the transgression, and also fines upward.

2. A regressive facies sequence coarsens in the direction of the regression, and also coarsens upward.

What causes transgressions and regressions?

1. Worldwide rises and falls in sea level (eustatic changes), perhaps related to climatic change.

2. Tectonic uplift, isostatic rebound, or crustal subsidence.

3. Rapid sedimentation.

It is often difficult or impossible to determine the exact cause of a transgression or regression seen in the geologic record.  The cause may be worldwide or local.  The fact that there is a transgression or regression indicates an “apparent” sea-level change.

 The Stratigraphy of Unconformities

Recall that unconformities represent missing time due to:

1)      Periods of non-deposition.

2)      Periods of erosion.

The main types of unconformities are:

1. Disconformity

2. Angular unconformity

3. Nonconformity

4. Paraconformity

Unconformities vary from one location to another (just like rock formations and sedimentary facies).  In other words, some locations along the unconformity surface will represent more missing geologic time than others.

Unconformities may eventually disappear laterally and transition into a conformable sequence of strata.

Oil companies use large scale, unconformity bounded rock units called sequences to correlate rocks in a process called sequence stratigraphy.

Six major unconformity-bounded sequences are recognized worldwide in the Phanerozoic.  These sequences are not restricted to period or era boundaries.

The major sequences are believed to represent worldwide fluctuations in sea-level.

Mars Mission

Geology Degree

Geology Degree

As a geology graduate, your mastery in undertaking field and lab examinations joined with your team-working, correspondence and systematic abilities make you an alluring prospect for some businesses.

Definition of Geology

Geology is a science that studies the Earth and the materials that it’s made of. It looks at the rocks that the Earth is composed of, the structure of the earth’s materials, and the processes acting upon those materials that cause the Earth to evolve. Through the study of geology we can understand the history of the Earth. Geologists decipher evidence for plate tectonics, the evolutionary history of life, and the past climates the Earth has been through. Geology also includes the study of organisms that have inhabited the planet, and how they’ve changed over time.

Currently we use geology for mineral and hydrocarbon exploration, evaluating water resources, predicting natural hazards, finding remedies for environmental problems, providing insights into past climate change, and geotechnical engineering. Through geology degrees people can study geology, become a geologist, and use their knowledge to improve our Earth.

A Geology Degree

If you’re interested in studying geology, there are a few different degree options open to you in both undergrad and graduate education. The following are a few options:

• Bachelor of Arts in Geology: The BA in geology degree is intended for students who plan to pursue teacher certification, natural resource management, scientific or technical writing, and other fields that combine a strong liberal arts background with science training. BA classes may include earth materials, minerals, igneous and metamorphic rocks, oceanography, principles of astronomy, deformation of the Earth, sedimentary processes, earth surface processes, and field methods.
• Bachelor of Science in Geology: The BS in geology degree differs from the BA in that it has a strong mathematical component. It’s typically designed for students planning to pursue graduate study in geology, or work as a professional geologist. Courses may include: History of the Earth, Earth materials, deformation of the Earth, sedimentary processes, Earth surface processes, field methods, chemistry, physics, physics in electricity and magnetism, and calculus classes.
• Master of Science in Geology: This is a graduate degree in geology. Master programs are advanced geology degrees with a focus on geology classes. They typically come in both thesis and non-thesis options. Those who want to get a master’s in geology degree must have an undergraduate degree in geology or a closely related science field. Sometimes they’ll let applicants without a bachelor’s degree in geology to take pre-requisite classes before beginning a master’s program. Pre-requisite classes include: physical geology, mineralogy, paleobiology, petrography, geologic field methods, stratigraphy, igneous/metamorphic petrogenesis, structural geology, sedimentary petrogenesis, and introduction to geophysics.
• Doctorate in Geology: A PhD is the highest level of degree a person can get in geology. These programs are designed to develop creative scholarship and to prepare the student for a professional career in the geological sciences. Typically a person chooses a specialisation or focus such as geochemistry, geology, geophysics, planetary geology, minerals, or more. Students can be admitted into PhD programs with either a bachelor’s or master’s degree in geology. Depending on the previous degree earned, a PhD may take one to two years of study.

In all degree levels of geology, the goal is for students to master basic concepts and vocabulary in geology. Through these programs you’ll learn the following materials:

• Plate tectonics
• Origin and classification of rocks and minerals
• Geological time scale and how this relates to major events in the history of Earth and its life
• Geophysical properties of the Earth and crustal deformation
• Processes that shape the surface of the Earth
• Environmental hazards and issues

You’ll also be expected to:

• Develop skills in observing and recording geologic features and processes
• Develop competency in the interpretation of earth science data, including both qualitative and quantitative analyses
• Achieve competence in: locating and interpreting scientific literature,
• Giving oral presentations,
• Using computers at a level consistent with current professional practice
• Be able to express earth science concepts in writing

Specializations Within Geology

Not all geologists study the same thing. Since the Earth is so large there are many areas that a geologist can focus on. The following are the most common types of geology specialisations within geology degrees:

• Economic Geology: These geologists help locate and manage the Earth’s natural resources. These resources may include petroleum, coal, and minerals such as iron, copper, and uranium. Typically these resources are used for profit companies.
• Mining Geology: This is a common form of geology that focuses on extracting mineral resources from the Earth. Typically these resources are of economic interest. They may include gemstones, metals, and many minerals such as asbestos, perlite, mica, phosphates, zeolites, clay, pumice, quartz, silica, sulphur, chlorine, and helium.
• Petroleum Geology: These geologists study locations below the Earth’s surface where extractable hydrocarbons may be. They especially focus on petroleum and natural gas. Petroleum geologists also study the formation of sedimentary basins, where gas reservoirs are typically found. They look into the sedimentary and tectonic evolution, and the present-day positions of rocks within the basins.
• Engineering Geology: This is the application of geologic principles to engineering practice. The main purpose of engineering geology is to assure that the geologic factors affecting the location, design, construction, operation, and maintenance of engineering works are properly addressed in engineering and building projects. In civil engineering geological principles are used in order to analyse the mechanical principles of the material on which structures are built, or will be built. This allows tunnels to be built without collapsing, and bridges and skyscrapers to be built with sturdy foundations.
• Environmental Geology: Geology can be applied to various environmental problems. It can be applied in steam restoration, the restoration of brownfield, the understanding of the interactions between natural habitat and the geologic environment, and more.
• Hydrology Geology: Groundwater hydrology, or hydrogeology, is used to locate groundwater. This can often provide a ready supply of uncontaminated water and is especially important in arid regions. It’s also used to monitor the spread of contaminants in groundwater wells.
• Natural Hazards Geology: Geologists study natural hazards in order to enact safe building codes. They also make warning systems that are used to prevent loss of property and life amidst natural hazards. The kind of natural hazards that geologists’ study include: avalanches, earthquakes, floods, landslides, debris flows, river channel migration, avulsion, liquefaction, sinkholes, subsidence, tsunamis, and volcanoes.

Day in a Life of a Geologist

In general, geologists work to understand the history of our planet. As noted above, there are many different types of geology that geologists can focus on. The better understanding geologists have of the Earth’s history, the better they can foresee how events and processes of the past and present may influence the future. We rely on geologists to find a good supply of Earth’s products, such as natural gases and minerals. Every structure that we build we must know about the ground it sits on. Our food and fibre comes from soil, which we must understand to produce food. Protection against geologic hazards also depends on a geologists’ understanding of them.

Geologists are responsible for many important contributions to society. In fact, we rely on them in everyday life without even realising it. Geologists use a number of field, laboratory, and numerical modelling methods to decipher Earth history. Through geological investigations, geologists use primary information related to the study of rocks (petrology), sedimentary layers (stratigraphy), and positions of rocks and their deformation (structural geology). In many cases, geologists also study modern soils, rivers, landscapes, and glaciers. They investigate past and current life and biogeochemical pathways, and use geophysical methods to investigate the subsurface of the Earth. The following are the typical categories that geologist’s work fall under:

• Earth processes: The Earth is constantly evolving and changing due to natural disasters and climate change. These include landslides, earthquakes, floods and volcanic eruptions, all of which can be hazardous to people. Some geologists’ jobs are to understand these processes. Through understanding them they help build important structures where they won’t be damaged, or create maps of areas that have flooded in the past and that could be flooded in the future. These kinds of maps can be used in developing communities and housing areas. They can determine if certain houses need flood insurance or protection.
• Earth materials: Humans use the Earth’s materials on a daily basis. We use oil that’s produced from wells, metals produced from mines, water drawn from underground sources, and more. A geologist’s work could involve studying rocks that contain important metals, finding and planning mines that could produce metals, finding oil reservoirs, and more.
• Earth History: A current issue for our world is climate change. Many geologists are currently working to learn more about past climates of the Earth and how they’ve changed over time. Through understanding these patterns, geologists can understand our current climate change and what the future results of that change may be.

Job Options

Occupations specifically identified with your degree include:

• Building geologist 
• Geochemist 
• Geophysicist/field seismologist 
• Geoscientist 
• Hydrogeologist 
• Seismic mediator 
• Mudlogger 
• Wellsite geologist 

Occupations where your degree would be helpful include:

• Drilling Engineer
• Ecological specialist 
• Geophysical information processor 
• Minerals surveyor 

Keep in mind that numerous businesses acknowledge applications from graduates with any degree subject, so don't confine you're supposing to the employments recorded here.

Work Experience

Hands on work in both the UK and abroad is a key a portion of topography courses as it gives down to earth experience. A few courses offer a year out, either abroad or in industry, an awesome chance to expand your ability set and build up a system of contacts.

A few graduates decide to improve their capabilities and abilities by doing paid or intentional deal with fleeting natural tasks in the UK or abroad. Time getting work experience or shadowing can assist you with settling on choices about your future vocation and you'll see it inspiring when you apply your ability to tackle issues in an alternate connection.

Typical Employers

Numerous geography graduates enter callings specifically identified with their degree. Prominent parts incorporate investigation and creation, water supply, natural building and topographical looking over. Different regions incorporate ecological arranging, hydrogeology and contamination control. Average managers of geography graduates include: 

• the oil, gas and petroleum area; 
• the groundwater business; 
• ecological consultancies; 
• structural building and development organisations.

Skills for your CV

Considering geography you'll increase particular learning identified with your project of study and module decisions. The down to earth field work you do as a major aspect of your degree outfits you with ability in field and research facility examinations. 

Transferable aptitudes from your course include: 

• aptitudes in perception, information gathering, examination and elucidation; 
• the capacity to get ready, process and present information; 
• the capacity to handle data in a scope of distinctive mediums, e.g. literary, numerical, oral, graphical; 
• composed and verbal relational abilities; 
• report composing abilities; 
• critical thinking aptitudes and horizontal considering; 
• self-inspiration and flexibility; 
• team-working aptitudes and the capacity to deal with your own particular activity.

Further Study

Further study is a prevalent alternative for geography graduates. In case you're keen on getting into a specific field of geography, for example, mining building, designing topography or the minerals business, what about taking an applicable M.Sc course? 

For instance, taking a M.Sc in petroleum geoscience is a possibility for those needing to get into the petroleum business. Different cases of postgraduate courses include:

• petroleum engineering;
• petroleum geophysics;
• earth sciences;
• hydrogeology;
• waste management;
• nuclear decommissioning.

A little number of understudies proceed onto PhD. By learning at postgraduate level, you'll build up your authority information, research aptitudes and relational abilities.

Rock layers

Rock layers

In geology and related fields, a stratum (plural: strata) is a sedimentary rock layer or soil with inside reliable qualities that recognize it from different rock layers. The "stratum" is the crucial unit in a stratigraphic section and structures the study's premise of stratigraphy.

Characteristics of rock layers

Every rock layer is for the most part one of various parallel rock layers that lie one upon another, set around characteristic procedures. They may stretch out over a huge number of square kilometres of the Earth's surface. Strata are normally seen as groups of diverse shaded or contrastingly organized material uncovered in bluffs, street cuts, quarries, and waterway banks. Individual groups may fluctuate in thickness from a couple of millimetres to a kilometre or more. Every band speaks to a particular method of affidavit: stream residue, shoreline sand, coal bog, sand ridge, magma bed, and so on.

Naming of rock layers

Geologists study rock strata and sort them by the material of beds. Each particular layer is normally doled out to the name of sheet, generally in view of a town, stream, mountain, or locale where the arrangement is uncovered and accessible for study. For instance, the Burgess Shale is a thick introduction of dim, once in a while fossiliferous, shale uncovered high in the Canadian Rockies close Burgess Pass. Slight refinements in material in an arrangement may be portrayed as "individuals" (or now and again "beds"). Arrangements are gathered into "gatherings" while gatherings may be gathered into "supergroups".

Formation

An formation or geological formation is the basic unit of lithostratigraphy. An arrangement comprises of a sure number of rock strata that have an equivalent lithology, facies or other comparable properties. Developments are not characterized on the stone's thickness strata they comprise of and the thickness of distinctive formation can thus change broadly. 

The idea of formally characterized layers or strata is key to the geologic control of stratigraphy. Arrangements can be separated into individuals and are themselves regularly divided in gatherings.

Usefulness of formation

The definition and acknowledgement of formations permit geologists to correspond geologic strata crosswise over wide separations in the middle of outcrops and exposures of rock strata. 

Developments were at first depicted to be the crucial geologic time markers in view of relative ages and the law of superposition. The divisions of the land time scale were the formations depicted and put in sequential request by the geologists and stratigraphers of the eighteenth and nineteenth hundreds of years. 

Current modification of the geologic sciences has limited formations to lithologies, in light of the fact that lithological units are shaped by depositional situations, some of which may continue for a huge number of years and will transgress chronostratigraphic interims or fossil-based routines for relating rocks. For instance, the Hamersley Basin of Western Australia is a Proterozoic sedimentary bowl where up to 1200 million years of sedimentation is saved inside of the in place sedimentary stratigraphy, with up to 300 million years spoke to by a solitary lithological unit of grouped iron arrangement and shale. 

Geologic developments are typically sedimentary rock layers, yet might likewise be transformative rocks and volcanic streams. Molten nosy rocks are for the most part not separated into formations.

Defining lithostratigraphic formations

Formations are the main formal lithostratigraphic units into which the stratigraphic section all over ought to be partitioned totally on the premise of lithology. 

The difference in lithology between arrangements required to legitimize their foundation shifts with the multifaceted nature of the geography of an area and the point of interest required for geologic mapping and to work out its geologic history. 

Formations must have the capacity to be depicted at the size of geologic mapping honed in the area. The thickness of developments may run from not as much as a meter to a few thousand meters. 

Geologic arrangements are normally named for the geographic territory in which they were initially portrayed. 

Entirely, developments can't be characterized on whatever other criteria aside from essential lithology. Nonetheless, it is frequently helpful to characterize biostratigraphic units in light of paleontological criteria, chronostratigraphic units taking into account the stones' age, and chemostratigraphic units in view of geochemical criteria. 

Succession stratigraphy is an idea which challenges the thought of strict lithostratigraphic units by characterizing units in light of occasions in sedimentary bowls, for example, maritime relapses and transgressions. These groupings are a mix of chronostratigraphic units, connected by time, and depositional environment connected by the geologic occasions which happened around then, paying little respect to the grain size of the silt. 

The expression "formation" is regularly utilized casually to allude to a particular gathering of rocks, for example, those experienced inside of a sure profundity range in an oil well.

What is Earth made of?

What is Earth made of?

The proportions of major elements making up the mass of the whole Earth.

At this point, we leave our fantasy space voyage and turn our attention inward to the materials that make up the solid Earth, because we need to be aware of these before we can discuss the architecture of the Earth’s interior. Let’s begin by reiterating that the Earth consists mostly of elements produced by fusion reactions in stars and by supernova explosions. Only four elements (iron, oxygen, silicon, and magnesium) make up 91.2% of the Earth’s mass; the remaining 8.8% consists of the other 88 elements (figure above). The elements of the Earth comprise a great variety of materials.

• Organic chemicals. Carbon-containing compounds that either occur in living organisms or have characteristics that resemble compounds in living organisms are called organic chemicals. 
• Minerals. A solid, natural substance in which atoms are arranged in an orderly pattern is a mineral. A single coherent sample of a mineral that grew to its present shape is a crystal, whereas an irregularly shaped sample, or a fragment derived from a once-larger crystal or cluster of crystals, is a grain. 
• Glasses. A solid in which atoms are not arranged in an orderly pattern is called glass. 
• Rocks. Aggregates of mineral crystals or grains, or masses of natural glass, are called rocks. Geologists recognize three main groups of rocks. (1) Igneous rocks develop when hot molten (liquid) rock cools and freezes solid. (2) Sedimentary rocks form from grains that break off pre-existing rock and become cemented together, or from minerals that precipitate out of a water solution. (3) Metamorphic rocks form when pre-existing rocks change in response to heat and pressure. 
• Sediment. An accumulation of loose mineral grains (grains that have not stuck together) is called sediment. 
• Metals. A solid composed of metal atoms (such as iron, aluminium, copper, and tin) is called a metal. An alloy is a mixture containing more than one type of metal atom. 
• Melts. A melt forms when solid materials become hot and transform into liquid. Molten rock is a type of melt geologists distinguish between magma, which is molten rock beneath the Earth’s surface, and lava, molten rock that has flowed out onto the Earth’s surface. 
• Volatiles. Materials that easily transform into gas at the relatively low temperatures found at the Earth’s surface are called volatiles. 

The most common minerals in the Earth contain silica (a compound of silicon and oxygen) mixed in varying proportions with other elements. These minerals are called silicate minerals. Not surprisingly, rocks composed of silicate minerals are silicate rocks. Geologists distinguish four classes of igneous silicate rocks based, in essence, on the proportion of silica to iron and magnesium. In order, from greatest to least proportion of silica to iron and magnesium, these classes are felsic (or silicic), intermediate, mafic, and ultramafic. As the proportion of silica in a rock increases, the density (mass per unit volume) decreases. Thus, felsic rocks are less dense than mafic rocks. Many different rock types occur in each class, as will be discussed in detail in Chapters 4 through 7. For now, we introduce the four rock types whose names we need to know for our discussion of the Earth’s layers that follows. These are (1) granite, a felsic rock with large grains; (2) gabbro, a mafic rock with large grains; (3) basalt, a mafic rock with small grains; and (4) peridotite, an ultramafic rock with large grains.

Discovering the Earth’s Internal Layers

People have speculated about what’s inside our planet since ancient times. What is the source of incandescent lavas that spew from volcanoes, of precious gems and metals found in mines, of sparkling mineral waters that bubble from springs, and of the mysterious forces that shake the ground and topple buildings? In ancient Greece and Rome, the subsurface was the underworld, Hades, home of the dead, a region of fire and sulphurous fumes. Perhaps this image was inspired by the molten rock and smoke emitted by the volcanoes of the Mediterranean region. In the 18th and 19th centuries, European writers thought the Earth’s interior resembled a sponge, containing open caverns variously filled with molten rock, water, or air. In fact, in the popular 1864 novel Journey to the Centre of the Earth, by the French author Jules Verne, three explorers hike through interconnected caverns down to the Earth’s centre.
How can we explore the interior for real? We can’t dig or drill down very far. Indeed, the deepest mine penetrates only about 3.5 km beneath the surface of South Africa. And the deepest drill hole probes only 12 km below the surface of northern Russia compared with the 6,371 km radius of the Earth, this hole makes it less than 0.2% of the way to the centre and is nothing more than a pinprick. Our modern image of the Earth’s interior, one made up of distinct layers, is the end product of many discoveries made during the past 200 years.
The first clue that led away from Jules Verne’s sponge image came when researchers successfully measured the mass of the whole Earth, and from this information derived its average density. They found that the average density of our planet far exceeds the density of common rocks found on the surface. Thus, the interior of the Earth must contain denser material than its outermost layer and can’t possibly be full of holes. In fact, the mass of the Earth overall is so great that the planet must contain a large amount of metal. Since the Earth is close to being a sphere, the metal must be concentrated near the centre. Otherwise, centrifugal force due to the spin of the Earth on its axis would pull the equator out, and the planet would become a disk. (To picture why, consider that when you swing a hammer, your hand feels more force if you hold the end of the light wooden shaft, rather than the heavy metal head.) Finally, researchers realized that, though molten rock occasionally oozes out of the interior at volcanoes, the interior must be mostly solid, because if it weren't, the land surface would rise and fall due to tidal forces much more than it does.

An early image of Earth’s internal layers.

Eventually, researchers concluded that the Earth resembled a hard-boiled egg, in that it had three principal layers: a not-so-dense crust (like an eggshell, composed of rocks such as granite, basalt, and gabbro), a denser solid mantle in the middle (the “white,” composed of a then-unknown material), and a very dense core (the “yolk,” composed of an unknown metal) (figure above). Clearly, many questions remained. How thick are the layers? Are the boundaries between layers sharp or gradational? And what exactly are the layers composed of?

Clues from the Study of Earthquakes: Refining the Image

Faulting and earthquakes.

When rock within the outer portion of the Earth suddenly breaks and slips along a fracture called a fault, it generates shock waves (abrupt vibrations), called seismic waves, that travel through the surrounding rock outward from the break. Where these waves cause the surface of the Earth to vibrate, people feel an earthquake, an episode of ground shaking. You can simulate this process, at a small scale, when you break a stick between your hands and feel the snap with your hands (figure above).

In the late 19th century, geologists learned that earthquake energy could travel, in the form of waves, all the way through the Earth’s interior from one side to the other. Geologists immediately realized that the study of earthquake waves travelling through the Earth might provide a tool for exploring the Earth’s insides, much as ultrasound today helps doctors study a patient’s insides. Specifically, laboratory measurements demonstrated that earthquake waves travel at different velocities (speeds) through different materials. Thus, by detecting depths at which velocities suddenly change, geoscientists pinpointed the boundaries between layers and even recognized subtler boundaries within layers. For example, such studies led geoscientists to subdivide the mantle into the upper mantle and lower mantle, and subdivide the core into the inner core and outer core.

Pressure and Temperature Inside the Earth

In order to keep underground tunnels from collapsing under the pressure created by the weight of overlying rock, mining engineers must design sturdy support structures. It is no surprise that deeper tunnels require stronger supports: the downward push from the weight of overlying rock increases with depth, simply because the mass of the overlying rock layer increases with depth. In solid rock, the pressure at a depth of 1 km is about 300 atm. At the Earth’s centre, pressure probably reaches about 3,600,000 atm. Temperature also increases with depth in the Earth. Even on a cool winter’s day, miners who chisel away at gold veins exposed in tunnels 3.5 km below the surface swelter in temperatures of about 53°C (127°F). We refer to the rate of change in temperature with depth as the geothermal gradient. In the upper part of the crust, the geothermal gradient averages between 20°C and 30°C per km. At greater depths, the rate decreases to 10°C per km or less. Thus, 35 km below the surface of a continent, the temperature reaches 400°C to 700°C, and the mantle-core boundary is about 3,500°C. No one has ever directly measured the temperature at the Earth’s centre, but calculations suggest it may exceed 4,700°C, close to the Sun’s surface temperature of 5,500°C.

Unconventional reserves of Hydrocarbons

Unconventional reserves of Hydrocarbons

Oil that can be extracted from reserves in the porous and permeable reservoir rocks of oil traps. Such reserves have come to be known as conventional reserves, because accessing them uses technology that has been around for years. In the past 10 to 15 years, energy companies have begun to increase their focus on extracting hydrocarbons from unconventional reserves, meaning reserves that had previously been left in the ground or disposed of because they cannot be tapped without using new technologies. Let’s look at a few examples of these reserves. 

Natural Gas 

Natural gas consists of volatile, short-chain hydrocarbon molecules (methane, ethane, propane, and butane). Gas burns more cleanly than oil, in that combustion of gas produces only CO2 and water, while the burning of oil not only produces CO2 and water, but also complex organic pollutants. Thus, natural gas has become the preferred fuel for home cooking and heating, and in some localities, for electricity production. It can also be used to run cars and trucks, if the vehicles have been appropriately modified. Natural gas has not yet been used as widely as other hydrocarbons, because gas transportation, which requires high-pressure pipelines or special ships, is quite expensive. But its use is increasing rapidly. As we have seen, gas often occurs in association with oil. Unfortunately, at many oil wells, it is not economical to capture and transport the gas, so this gas vents from a pipe and is burned in a flare where it enters the air. (In localities where rock has been heated to temperatures higher than the oil window, reservoir rock may contain only gas and there may be enough to be worth pumping and capturing by conventional means.) Recently, the use of directional drilling and hydraulic fracturing has made it possible to extract large quantities of gas directly from source rocks. Large reserves of such shale gas underlie states in the northeastern United States, and are currently being drilled to provide energy for east-coast cities (Box 12.2). Intense exploration for shale gas reserves has begun worldwide.

Tar Sands (Oil Sands) 

So far, we’ve focused our discussion on hydrocarbon reserves that can be pumped from the subsurface in the form of a liquid or gas. But in several locations around the world, most notably Alberta (in western Canada) and Venezuela, vast reserves of very viscous, tar-like “heavy oil” exist. This heavy oil, known also as bitumen, has the consistency of gooey molasses, and thus cannot be pumped directly from the ground. It fills the pore spaces of sand or of poorly cemented sandstone, constituting up to 12% of the sediment or rock volume. Sand or sandstone containing such high concentrations of bitumen is known as tar sand or oil sand. Production of usable oil from tar sand is difficult and expensive, but not impossible. It takes about 2 tons of tar sand to produce one barrel of oil. Oil companies mine near-surface deposits in vast open-pit mines and then heat the tar sand in a furnace to extract the oil. Producers then crack the heavy oil molecules to produce smaller, more usable molecules. Trucks dump the drained sand back into the mine pit. To extract oil from deeper deposits of tar sand, oil companies drill wells and pump steam or solvents down into the sand to liquefy the oil enough so that it can be pumped out. 

Oil Shale 

Vast reserves of organic shale have not been subjected to temperatures of the oil window, or if they were, they did not stay within the oil window long enough to complete the transformation to oil. Such rock still contains a high proportion of kerogen. Shale that contains at least 15% to 30% kerogen is called oil shale. Lumps of oil shale can be burned directly and thus have been used as a fuel since ancient times. In general, however, energy companies produce liquid oil from oil shale. The process involves heating the oil shale to a temperature of 500nC; at this temperature, the shale decomposes and the kerogen transforms into liquid hydrocarbon and gas. As is the case with tar sand, production of oil from oil shale is possible, but very expensive.

Oil Exploration and Production

Oil Exploration and Production

Birth of the Oil Industry 

In the United States, during the first half of the 19th century, people collected “rock oil” (later called petroleum, from the Latin words petra, meaning rock, and oleum, meaning oil) at seeps and used it to grease wagon axles and to make patent medicines. But such oil was rare and expensive. In 1854, George Bissel, a New York lawyer, came to the realization that oil might have broader uses, particularly as fuel for lamps, to replace increasingly scarce whale oil. Bissel and a group of investors contracted Edwin Drake, a colourful character who had drifted among many professions, to find a way to drill for oil in rocks beneath a hill near Titusville, Pennsylvania, where oily films floated on the water of springs. Using the phony title “Colonel” to add respectability, Drake hired drillers and obtained a steam-powered drill. Work was slow and the investors became discouraged, but the very day that a letter arrived ordering Drake to stop drilling, his drillers found that the hole, which had reached a depth of 21.2 m, had filled with oil. They set up a pump, and on August 27, 1859, for the first time in history, pumped oil out of the ground. No one had given much thought to the question of how to store the oil, so workers dumped it into empty whisky barrels. This first oil well yielded 10 to 35 barrels a day, which sold for about $20 a barrel (1 barrel equals 42 gallons). Within a few years, thousands of oil wells had been drilled in many states, and by the turn of the 20th century, civilization had begun its addiction to oil. Initially, most oil went into the production of kerosene for lamps. Later, when electricity took over from kerosene as the primary source for illumination, gasoline derived from oil became the fuel of choice for the newly invented automobile. Oil was also used to fuel electric power plants. In its early years, the oil industry was in perpetual chaos. When “wildcatters” discovered a new oil field, there would be a short-lived boom during which the price of oil could drop to pennies a barrel. In the midst of this chaos, John D. Rockefeller established the Standard Oil Company, which monopolized the production, transport, and marketing of oil. In 1911, the Supreme Court broke down Standard Oil into several companies including Exxon (Esso), Chevron, Mobil, Sohio, Amoco, Arco, Conoco, and Marathon some of which have recombined in recent decades. Oil became a global industry governed by the complex interplay of politics, profits, supply, and demand. 

The Modern Search for Oil 

Wildcatters discovered the earliest oil fields either by blind luck or by searching for surface seeps. But in the 20th century, when most known seeps had been drilled and blind luck became too risky, oil companies realized that finding new oil fields would require systematic exploration. The modern-day search for oil is a complex, sometimes dangerous, and often exciting procedure with many steps. Source rocks are always sedimentary, as are most reservoir and seal rocks, so geologists begin their exploration by looking for a region containing appropriate sedimentary rocks. Then they compile a geologic map of the area, showing the distribution of rock units. From this information, it may be possible to construct a preliminary cross section depicting the geometry of the sedimentary layers underground as they would appear on an imaginary vertical slice through the Earth.

To add detail to the cross section, an exploration company makes a seismic-reflection profile of the region. To obtain a seismic profile, a special vibrating truck or a dynamite explosion sends seismic waves (shock waves that move through the Earth) into the ground. The seismic waves reflect off contacts between rock layers, just as sonar waves sent out by a submarine reflect off the bottom of the sea. Reflected seismic waves then return to the ground surface, where sensitive instruments (geophones) record their arrival. A computer measures the time between the generation of a seismic wave and its return, and from this information defines the depth to the contacts at which the wave reflected. With such information, the computer constructs an image of the configuration of underground rock layers and, in some cases, can “see” reserves of oil or gas. 

Drilling and Refining 

If geological studies identify a trap, and if the geologic history of the region indicates the presence of good source rocks and reservoir rocks, geologists make a recommendation to drill. (They do not make such recommendations lightly, as drilling a deep well may cost over $50 million.) Once the decision has been made, drillers go to work. These days, drillers use rotary drills to grind a hole down through rock. A rotary drill consists of a pipe tipped by a rotating bit, which is a bulb of metal studded with hard metal prongs. As the bit rotates, it scratches and gouges the rock, turning it into powder and chips. Drillers pump “drilling mud,” a slurry of water mixed with clay and other materials, down the center of the pipe. The mud flows down, past a propeller that rotates the drill bit, and then squirts out of holes at the end of the bit. The extruded mud cools the bit head, which otherwise would heat up due to friction as it grinds against rock, then flows up the hole on the outside of the drill pipe. As it rises, the mud carries “rock cuttings” (fragments of rock that had been broken up by the drill bit) up and out of the hole. Mud also serves another very important purpose its weight counters the pressure of the oil and gas in underground reservoir rocks. By doing so, it prevents hydrocarbons from entering the hole until drilling has been completed, the hole has been “finished” (by removing the drill pipe and sealing the walls of the drill hole with concrete), and the hole has been capped. Were it not for the mud, the natural pressure in the reservoir rock would drive oil and/or gas into the hole. And if the pressure were great enough, the hydrocarbons would rush up the hole and spurt out of the ground as a gusher or blowout. Gushers and blowouts can be disastrous, because they spill oil onto the land and, in some cases, ignite into an inferno. Early drilling methods could produce only vertical drill holes. But as technology advanced, drillers developed methods to control the path of the drill bit so the hole can curve and become diagonal or even horizontal. Such directional drilling has become so precise that a driller, using a joystick to steer the bit, and sensors that specify the exact location of the bit in 3-D space, can hit an underground target that is only 15 cm wide from a distance of a few kilometres. Drillers use derricks (towers) to hoist the heavy drill pipe. To drill in an offshore hydrocarbon reserve, one that occurs in strata beneath the continental shelf, the derrick must be constructed on an offshore-drilling facility. These may be built on huge towers rising from the sea floor, or on giant submerged pontoons. Using directional drilling, it’s possible to reach multiple targets from the same platform. On completion of a hole, workers remove the drilling rig and set up a pump. Some pumps resemble a bird pecking for grain; their heads move up and down to pull up oil that has seeped out of pores in the reservoir rock into the drill hole. You may be surprised to learn that simple pumping gets only about 30% of the oil in a reservoir rock out of the ground. Thus oil companies may use secondary recovery techniques to coax out more oil (as much as 20% more). For example, a company may drive oil toward a drill hole by forcing steam into the ground nearby. The steam heats the oil in the ground, making it less viscous, and pushes it along. In some cases, drillers create artificial fractures in rock around the hole by pumping a high-pressure mixture of water, various chemicals, and sand into a portion of the hole. This process, called hydrofracturing (or “fracking”) creates new fractures and opens up pre-existing ones. The sand left by the fracturing fluid keeps the cracks from closing tightly, so they remain permeable. The fractures provide easy routes for the oil to follow from the rock to the well. Once extracted directly from the ground, “crude oil” flows first into storage tanks and then into a pipeline or tanker, which transports it to a refinery. At a refinery, workers distil crude oil into several separate components by heating it gently in a vertical pipe called a distillation column. Lighter molecules rise to the top of the column, while heavier molecules stay at the bottom. The heat may also “crack” larger molecules to make smaller ones. Chemical factories buy the largest molecules left at the bottom and transform them into plastics.

Where Does Oil Occur? 

Reserves are not randomly distributed around the Earth. Currently, countries bordering the Persian Gulf contain the world’s largest reserves in 25 supergiant fields. In fact, this region has almost 60% of the world’s reserves. Reserves are specified in barrels (bbl); 1 bbl 42 gallons 159 liters. Why is there so much oil in the Middle East? Much of the region that is now the Middle East was situated in tropical areas between latitude 20n south and 20n north between the Jurassic (135 Ma) and the Late Cretaceous (65 Ma). Biological productivity was very high in these tropical regions, so the muds that accumulated there were very organic rich and lithified to become excellent source rocks. Thick layers of sand buried the source rocks and eventually became porous sandstones that make excellent reservoir rocks. Later, mountain-building processes folded the layers into large anticlines, which are excellent traps. The Middle East is not the only source of oil. Reserves also occur in sedimentary basins formed along passive continental margins, such as the Gulf Coast of the United States and the Atlantic Coasts of Africa and Brazil, as well as in intracratonic and foreland basins within continents.